The invention relates to a fluid fuel heating system intended especially for vehicles and comprising a burner unit provided with a combustion air blower driven by an electric motor and an incandescent plug igniting the fuel-air mixture only during the starting period.
In known heating systems of this type (German application (Offenlegungsschrift) No. 2,251,184) it is sometimes desirable to decrease the combustion air blower rpm to reduce the heat output and the noise level. A two-stage motor with change-over winding can be used for this purpose, but such motors are expensive. In another aspect of this known arrangement, a series resistor is placed in the motor circuit to reduce the rpm. However, such a series resistor constitutes an additional element which complicates the arrangement and increases its size.
Thus, an object of the present invention is to provide a heating system in which the rpm of the combustion air blower can be selectively decreased by an especially simple and economical means.
According to preferred embodiment of the invention this object is achieved by selectively connecting the incandescent plug as a series resistor to the electric motor driving the combustion air blower after completion of the starting period. Therefore, according to the invention, the incandescent plug which is normally present but heretofore necessary only during the starting period, is utilized when needed as a series resistor to reduce the blower rpm following the starting period. Thus, an additional element in the form of a special series resistor is eliminated.
For a fixed blower diameter the noise of the combustion air blower is determined substantially by the rpm. As the rpm decreases the heat output also drops, and the power consumption of the heating system diminishes. Therefore, the above arrangement can be considered as a partial load circuit. Such a partial load circuit can be used with advantage especially when one of the following conditions is of high importance:
Heating of small rooms or at mild temperatures when, except for the initial heat-up phase, a low heat output is sufficient;
Heating of residential or bed rooms in which a low noise level is highly desired; and
Continuous operation of a battery-supplied heating system with the lowest possible power consumption.
In an embodiment of the invention a relay is provided to connect the incandescent plug as a series resistor in the motor circuit, and a selectively closable switch contact and one contact of a combustion monitoring thermostat are contained in an actuating circuit of said relay. The contact of the combustion monitoring thermostat assures that the incandescent plug operates at full power even when the heating system is switched to partial load.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which show for purposes of illustration only, several embodiments in accordance with the present invention.